


The Sensational Shrinking Sans

by snuggleskull



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, BittyBones, Gen, Micro, Muffins of Unusual Size and a toga, Stuffing, Underswap Papyrus, Underswap Sans, but not really bittybones? there is a bitty sized Sans tho, undertum - Freeform
Language: English
Status: In-Progress
Published: 2016-08-12
Updated: 2016-08-20
Packaged: 2018-08-08 06:52:15
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 3,411
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7747465
Author URL: https://archiveofourown.org/users/snuggleskull/pseuds/snuggleskull
Summary: <blockquote class="userstuff">
              <p>Underswap Sans learns about the importance of reading labels and Dr. Undyne’s cure for bellyaches.</p>
            </blockquote>





	1. Chapter 1

A violent pounding on Undyne’s lab door startled the scientist from her Mew Mew Kissy Cutie marathon. There was only one monster Undyne knew that knocked as if they were trying to break down the door. She paused her anime and rushed to the lab entrance.

Right before she hit the button that would open the door, she took a deep breath and smoothed out her rumpled lab coat. Now thoroughly prepared, Undyne hit the button. The lab door slid open with a hiss.

“A- Alphys! It’s so g-good to see you!” Undyne shyly smiled at the burly lizard monster that stood outside her lab. However, the serious expression on the captain’s scarred face made Undyne falter. “W-what’s wrong?”

Alphys opened her mouth, but closed it without saying anything. She looked down at her clawed fist and then thrust her hand toward Undyne. The timid scientist took an involuntary step back.

“MWEHEHEH! Greetings, Doctor Undyne! Surely you recognize me, the Sensational Sans!” A familiar voice squeaked. In Alphys’ hand sat an impossibly tiny skeleton.

“We have a small problem.” Alphys growled.

~

Sans stood on the lab counter. He wore only his blue bandana tied around him like a toga. He was smaller than most of Undyne’s colorful beakers. That meant he couldn’t be more than five inches tall.

The small skeleton let Undyne inspect him. She recorded his height and weight and took some other measurements. He was glad that Alphys had already returned to her royal guard duties so that she didn’t see him in such a helpless state.

He suffered through Dr. Undyne’s further poking and prodding as she excitedly rambled about scientific stuff that Sans didn’t really understand. Papyrus probably would’ve been able to explain it to him, his brother was smart like that. When she was finally finished, the tiny skeleton was tired and hungry.

“So you can return me to my naturally glorious stature?” Sans asked optimistically.

Undyne shook her head and blushed. “S-sorry, Sans. I can’t counteract the effects of the Shrinking Sauce.”

“You mean I’m going to be this size forever! How will I ever become a member of the royal guard?” Sans wailed. Bright blue tears gathered at the corners of his eye sockets.

“N-No!” Undyne flapped her arms. She hadn’t meant to upset him. The sight of tears made her panic. “The effects should wear off naturally in a few days!”

“Oh, I see.” Sans sat down and curled his short arms around his knees. He rubbed his face against them, feeling quite embarrassed.

A light touch on his skull made him peak up. Undyne looked concerned as she gently pet Sans’ skull with a blue scaly finger. Sans sniffled; this was all his fault.

“It’s okay. You didn’t hurt anyone. N-next time you know to read the directions more thoroughly.” Undyne consoled him.

Sans knew she was right. Undyne was a super smart scientist after all. But he still felt silly for drinking an entire bottle of Shrinking Sauce, rather than dabbing just a bit behind his ears. If only he had read the directions!

All of the excitement had drained his magic too. He hadn’t eaten since breakfast and now it was late afternoon. Eating would definitely help Sans feel better about everything.

“If you had any food to spare, it would be very much appreciated Dr. Undyne.” Sans politely asked.

Undyne slapped her forehead, just barely missing her glasses.

“Geez, I’m such a terrible host! Let’s get you some food.” She scooped up her friend and took him to the kitchen. The dizzy little skeleton was set down on the counter while Undyne began looking through her cupboards.

“I guess it would be difficult for you to eat ramen.” Undyne mumbled as she stared at shelves full of cheaply packaged instant noodles.

“Do you have tacos?” Sans asked. Undyne’s red ponytail shook back and forth with her back still turned to him. The skeleton sighed, discouraged.

A squawk of delight made Sans jump. Undyne was digging through the refrigerator. She emerged from it and victoriously presented Sans with a muffin that was easily twice his size.

“You like blueberries, r-right?” Undyne set the muffin on the counter next to Sans. His wide eyes seemed to sparkle as he approached the huge confection.

“INDEED! Thank you for your most generous offer, Dr. Undyne!” The skeleton smiled brightly up at the aquatic monster. Undyne’s blush went unnoticed as Sans tucked in. Sans was a very cute skeleton to begin with. Now that he was the size of a muffin, Sans was beyond adorable!

Sans broke off pieces of muffin with his miniscule phalanges, leaving crumbs scattered around him. Each bite that he brought to his mouth made him feel more energized. The pouch of blue magic that rested in his abdominal cavity below his ribs steadily expanded. The fullness of his stomach always comforted Sans. When he was full, he was safe and happy.

About halfway through the muffin, Sans’ pace slowed. He wasn’t hungry any more. But he knew that his tummy could grow to hold the rest of it. There was no longer a need to eat, but Sans still wanted more.

Undyne had been quietly watching Sans. It wasn’t everyday that she got to observe a skeleton monster eating. Although the most interesting process was covered by the makeshift bandana toga. The amount of food he ate was very impressive considering his current size.

A less scientific thing that Undyne observed was the endearing way the skeleton cheerfully munched his handfuls of muffin. She hadn’t wanted to interrupt his meal, but she noticed a definite belly bulge beneath the bandana and Sans was eating much more slowly. Surely he was full by now and just didn’t want to seem rude by not eating it all.

“I-if you want, I can setup some anime for you to watch? And you can keep the rest of the muffin for a snack?” Undyne suggested.

Sans stared longingly at the other half of the muffin. He wanted to eat it right now. But he would polite and show some self restraint. “That sounds like fun! Can we watch the one with the robots?”

~

Sans groaned pitifully and held his aching stomach. Anime played on the monitor that he lay in front of. Undyne had gone to check on her experiments some time ago, leaving Sans alone with his muffin.

The colorful animation had held the skeleton’s attention for awhile. But soon he was tired of craning his neck to watch the tall screen. With his one distraction exhausted, his self restraint hadn’t lasted long.

He had slowly pushed morsels of muffin into his mouth, enjoying the stretch of his magic as it grew. The blueberries left faint blue stains on his jaw bone. His blue belly was firm and swollen by the time Sans had realized that the muffin was gone.

“Gollie! I ate it all. Truly, there is no obstacle too great for the Sensational Sans. No matter my size!” Sans declared. He raised his hand to attempt a dramatic pose, but the sudden motion upset his stomach. There was an uncomfortable bubbly sensation that traveled up to his mouth.

“MUUURP.” Sans let out a loud burp. He looked around frantically to make sure no one had seen him. He laid down, ashamed of his vulgarity. His boney hands patted the round distended ball of magic soothingly.

“Sans? Are you o-okay? I thought I heard something?” Undyne walked back into the room, curiously searching for the source of the strange noise. Sans felt another buildup of pressure trying to force its way out. He gritted his teeth and held it in. He tried to grin, willing Undyne to leave quickly.

“Sans? Is everything alright?” Undyne stared at him. Sans nodded. Undyne looked unconvinced.

“Why aren’t you talking? O-oh no! D-did you lose your voice?! I’m going to call Alphys. What will I tell Papy-”

“UURRRPPP.” Sans belched. He covered his mouth with both of his hands. His bones trembled in mortification.

Undyne’s glasses slid down as she looked disbelievingly at the small skeleton. Did he just burp? How could a skeleton even do that?! Was this a side effect of the Shrinking Sauce? Had it interacted badly with the muffin? Now that she thought about it, where was the rest of muffin? It was gone, only the paper wrapper remained.

“Did you eat the entire muffin?” Undyne asked in awe. Sans grimaced guiltily. His stomach gurgled and he reached down to stroke it.

“Sorry for my rudeness. My stomach is struggling to overcome a deceptively difficult opponent.” Sans whimpered. Undyne took a moment to translate in her head.

“S-so, you have a bellyache? Can I help?” Undyne watched as Sans’ tiny hands moved over the bulge that must be his stomach. She had an idea. Slowly, she picked Sans up, cradling him in her scaly hand. Her thumb hovered hesitantly above his tummy.

“Is this okay?” Undyne couldn’t tell what Sans was thinking, but with a bashful nod of his skull, he gave her permission to rub his belly. His cheekbones blushed blue to match Undyne’s skin as she massaged him gently with her thumb.

Sans couldn’t help sighing and moaning in relief as Undyne applied steady warm pressure to his engorged magic middle. It felt much better when another monster rubbed the pain away. Undyne was flushed, but she continued to pet the taut surface of Sans’ magic.

Just when both of them had relaxed into the peacefulness of the moment, an alarm blared. Undyne yelped and dropped Sans into her lab coat pocket without thinking. She ran to see what had gone wrong in another part of the lab.

Sans was jostled about in her pocket. He couldn’t gain any stability. The constant shaking as he bounced against Undyne’s side made him feel queasy. The magic inside of him felt like it was going to explode.

His cry of distress alerted Undyne to the situation. But she had to concentrate on fixing her experiment. She compromised by sticking a hand in her pocket to comfort the tiny monster. Her fingers resumed their even warm strokes.

Sans mewled. His bones had stiffened in discomfort. However, Undyne’s fingers sapped the tension out of him. His stomach settled under her ministrations.

It was dark and warm in Undyne’s pocket. When he yawned, his jawbone almost unhinged. It had been a very long day. His eye sockets drifted shut and the small skeleton fell soundly asleep.


	2. Chapter 2

**Summary for the Chapter:**

> Papy worries too much. Sans endures pokes and puns. Undyne gets the last laugh.

Papyrus stepped through the darkness and into his bedroom. To anyone in Hotland, it appeared as if the lanky skeleton had just ducked behind his corn dog stand. But when he stood up with a lazy grin, Papyrus was in his home in Snowdin. For being so tall, Papyrus knew some useful shortcuts.

He pulled off his baggy orange hoodie and threw it on the floor. The trash tornado that perpetually spun in the corner of his room pulled its latest victim into the vortex. He picked out another one of his many nearly identical hoodies from his closet and shrugged it on. Whether or not his clothes were clean didn’t really matter to Papyrus, but his brother Sans always complained when he came home smelling like grease and condiments.

It was Taco Tuesday, so Papyrus was on his best behavior. His boisterous brother made tacos everyday, but on Tuesdays he insisted that they make time to eat together. Even though Sans was busy training to join the royal guard and working as a sentry, he said it was important to do “brotherly bonding”.

So it was strange when Papyrus poked his head out into the hall and didn’t hear Sans cheerfully humming as he cooked.

“Hey, bro! You home?” His voice echoed through the still house. Black sneakers propelled the large skeleton forward and down the stairs with uncanny speed. It only took him a moment to see that Sans wasn’t in the kitchen. His pink apron still hung neatly on the hook next to the refrigerator. The brightly colored message board on the freezer door was blank.

The pulse of his soul set Papyrus on edge. Where was his brother? No monster in the underground would purposefully hurt him. But what if there had been an accident?

Anyone would’ve cleared out of Papyrus’ way if they had seen the serious set of his jawbone. Determined to find Sans, he jerked open the front door. A pile of clothes on the welcome mat made him stop short. It was Sans’ battle body.

“What in the stars?” Papyrus swore as he picked up his brother’s clothes. The bright blue bandana which Sans always wore was conspicuously absent and there was a note pinned to the shirt.

 

_Your brother won’t need these for awhile. He’s with Undyne at the lab. Don’t worry, you huge dork._

**_ALPHYS_ **

 

For some reason, Papyrus didn’t feel reassured after reading the captain’s message. He knew Sans wouldn’t miss Taco Tuesday without telling him first. Something fell out of Sans’ pants and clattered to the ground. It was his cellphone.

Papyrus’ soul leaped into his non-existent throat. Unsteady phalanges picked up the sticker covered cellphone. Sans’ phone straps jingled merrily as Papyrus clutched it tightly. Sans would never forget his phone.

Magic began to gather in his eye socket. There was something going on and he was going to get some answers. One moment, the usually laid back skeleton loomed in the doorway, and the next he was gone.

~

Sans softly moaned as he began to wake up. He squeezed his eye sockets shut and curled up, trying to drift back to sleep. It was so warm and cozy in the pocket of Undyne’s lab coat. His blue stomach, full of magic and muffin, made the tiny little skeleton contentedly lethargic. His boney hand rubbed against the soft fabric of the bandana that covered his distended belly. Sans smiled. His stomach ache had been soothed away by the steady strokes of Undyne’s fingers.

Unfortunately, Sans’ pleasant doze didn’t last long.

“Where’s my bro?” A familiar voice growled. Sans felt Undyne jump in shock and let out a yelp.

“P-P-Papyrus! Don’t just p-pop up behind me!” Undyne stuttered.

“Alphys left a note and Sans’ phone on my doorstep with no explanation. I’m a skeleton, but you know I’m not a bonehead. What’s going on?” Papyrus sounded strange. Held so close to Undyne’s body, Sans felt her trembling. For a moment, Sans hesitated to reveal himself. It was bad enough that Alyphs, the captain of the royal guard, saw him like this - all small and defenseless. It seemed even more embarrassing for Papyrus to find out. But a royal guard would never be cowardly and hide, especially from their own mistakes! So as Undyne sputtered, Sans’ skull peaked out of her pocket.

“Papy! I’m quite alright, brother!” Sans squeaked. He sleepily waved up at Papyrus with one hand and covered his wide yawn with the other.

“Excuse me.” A light blue blush tinted the mini-sized skeleton’s cheekbones. “Even future famous royal guards take naps now and again!” Normally Papyrus’ eye sockets rested only half-way open, but now they appeared to bulge as he stared at Sans.

“Bro?” Papyrus slowly kneeled down.

“MWEHEHEH, indeed!” Sans couldn’t resist laughing at his brother’s flabbergasted expression. It wasn’t often that Papyrus lost his cool composure.

He stretched his thin arms up and out. Papyrus carefully extended his hands forward. Sans grabbed one of his brother’s long phalanges and lifted himself out of Undyne’s cozy pocket. The bandana clad handful of a skeleton stood up in the protective cradle of his brother’s bones.

No one said anything for a moment. Papyrus opened and closed his jaw several times, unsure where to start. Sans smiled and stretched, enjoying his brother’s surprise.

“So, this is what you were all keeping on the down **low**?” Papyrus finally asked.

“Brother! Now is not the time for your ridiculous puns!” Sans indignantly stomped his foot, but he was so light that Papyrus barely felt it at all. Papyrus’ expression relaxed into a mischievous look.

“Now you don’t have to be **short** with me, bro.” Undyne’s muffled giggle-snort did not go unnoticed. Sans turned around and looked desolately at the doctor as she tried to stifle her laughter.

“No! Not you too! Dr. Undyne do not be swayed by my brother’s terrible attempts at comedy!” Sans implored. Papyrus looked Undyne dead in the eye and then down at the small fuming skeleton.

“Looks like I have a mu **tiny** on my hands.” Sans’ jaw dropped. He spun around to face his brother again as Undyne guffawed uncontrollably. Papyrus winked at him. “Bro, you think my humor is **low** brow, but you’re smiling.” Papyrus continued. Undyne gasped for breath in the background.

“I am and I hate it!” Sans grumbled and crossed his arms. One of Papyrus’ phalanges playfully prodded the bulge in Sans’ middle. He knew it was a store of excess magic because sometimes the space between his rib cage and pelvis would fill with leftover magic too.

As lazy as he was, Papyrus didn’t need to carry a lot of extra. Since Sans’ spent most of his days running around, he ate everyday and maintained a reserve. However, Papyrus had never seen his brother’s stomach extend to such a generous and poke-able size. It was warm and had an elastic give to it.

Sans swatted at his brother’s finger halfheartedly. Papy’s puns were awful, but Sans thought that he partly deserved them today. He felt bad that his brother had been so worried. It was his job to look after Papyrus. Who knew what would happen to the lazybones if he didn’t have such a cool brother taking care of him?

“How did this happen?” Papyrus asked. Sans hemmed and hawed, reluctant to tell his brother about his silly mistake. He glanced up at Undyne. She paused before realizing what her small skeleton friend wanted.

“O-oh! Well, Sans drank a bottle of Shrinking Sauce!” Undyne explained. “It’s not really supposed to be ingested, but Sans seems to be fine. The affects should wear off in a few days…” She trailed off.

Papyrus’ eyebrow ridges furrowed. His questioning gaze landed on Sans. The little skeleton shifted in his palms without meeting his look.

“Even future famous royal guards can forget to read labels, Papy! How was I supposed to know that you dab Shrinking Sauce behind your ears. It’s name is intentionally misleading!” Sans threw up his hands in frustration.

Papyrus shook his head. He let out a long a breath. Undyne tittered nervously.

“What?” Sans demanded.

“Bro,” Papyrus drawled. “We’re skeleton monsters.”

“Obviously, Papy.” Sans replied impatiently. Papyrus looked amused about something. “We don’t have ears.”

Sans froze. Little boney hands reached up to feel the smooth sides of his skull. His eye sockets widened in dawning horror.

“Sans?” Undyne asked, slightly concerned. Papyrus and Undyne jumped as Sans let out an anguished squeal.

“No!!! The Sensational Sans japed himself?!” He wailed. “Alphys must never know of my genius or I will never join the royal guard!”

“Hey, there’s no need to be **little** yourself, bro.” Papyrus teased, but Sans was too distraught to admonish his pun.

“Sorry, Sans.” Undyne comforted him. “I guess you could say you got the short end of the stick this time.”

The tiny skeleton covered his face and continued moaning. Papyrus stood up, resuming his slouched posture, and tucked his brother into his hoodie pocket for safekeeping.

Undyne would have thought him completely unassuming if she hadn’t seen his eye socket blazing with magic before Sans had popped out of her pocket.

“Thanks for taking care of him, Undyne. I’ll remember it.” Papyrus held out his hand. Undyne shivered. There was something unnerving about him, but she reached out to shake his hand anyway.

“It was no prob- **THBBPTHBPT!** ” Undyne blushed as Papyrus smirked and held up his hand, revealing a whoopee cushion.

“That joke never gets old. See you later, Doc.” With a mockingly jaunty salute, Papyrus turned on his foot and the skeleton brothers were gone.

Undyne took a deep breath and adjusted her glasses. She fished her phone out of her other pocket and looked at her new background, a picture of itty-bitty Sans sprawled out asleep in her hand. A shy toothy grin lit up her face. Sometimes being the Royal Scientist had its perks.

**Notes for the Chapter:**

> Whelp, this is part 2. I feeling like my style changed or is it the tone? IDK
> 
> thinking up puns is way too much fun :3

**Author's Note:**

> This is my first venture into undertum fic~ This chapter was inspired by marumaruowl on tumblr. It took me like 5 rewrites to figure out what POV I wanted to use *sigh* You can check out my blog at snuggleskull.tumblr.com >3>


End file.
